Robbe Gnat.. Scale Ducted-fan to Slope Soarer convex

The project is actually headed mainly, by a good friend, and owner of the original kit. To lay out the history of the model briefly, the kit was bought of the shelf by Anthony on a visit to the UK. The aircraft never really flew, being fraught with reception problems, and a thrust to weight ratio headache. After a good stint of hanger time , the fairly bedraggled fuselage was hauled out and restored under the new guise of being a scale slope soarer. Solid balsa elevons were fitted, with a Red Arrows livery scheme covering. The aircraft is almost ready for her maiden flight in the new attire. This will basicly take the form of a series of hand launches, at a spot that has a light gradient, to extend the flight time in order to get the bird trimmed/setup.
The big question at the moment, is where to put the Center of Gravity. It obviously Cant be in the same position as before, when she was running the ducted fan, as the lack thereof, will mean all of the force couples go out the window. Her highly tapered scale wings, and fairly uncambered aerofoil section, mean that eyeballing it is a little tricky.

Is there anyone who could offer some assist on the matter, and/or give any hints on the conversion itself, before we go throwing the little airey off a mountain.
Erik Larsen
Cape Town, ZA